catalysts Review C‐Homoscorpionate Oxidation Catalysts—
C-Homoscorpionate
Oxidation
Electrochemical and Catalytic Activity Catalysts—Electrochemical and Catalytic Activity
Review
Luísa M. D. R. S. Martins 1,2 Luísa
M. D. R. S. Martins 1,2
1
Chemical Engineering Department, Instituto Superior de Engenharia de Lisboa, Chemical Engineering Department, Instituto Superior de Engenharia de Lisboa,
Instituto Politécnico de Lisboa, R. Conselheiro Emídio Navarro, 1959‐007 Lisboa, Portugal; Instituto Politécnico de Lisboa, R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal;
[email protected]; Tel.: +351‐21‐8317226 2 Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, [email protected]; Tel.: +351-21-8317226
2
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1049–001 Lisboa, Portugal 1049–001 Lisboa, Portugal
Academic Editor: Georgiy B. Shulʹpin Academic
Editor: Georgiy B. Shul’pin
Received: 18 November 2016; Accepted: 23 December 2016; Published: 1 January 2017 Received: 18 November 2016; Accepted: 23 December 2016; Published: 1 January 2017
1
Abstract: A survey of the electrochemical properties of homoscorpionate tris(pyrazol‐1‐yl)methane Abstract: A survey of the electrochemical properties of homoscorpionate tris(pyrazol-1-yl)methane
complexes is presented. The relationship between structural features and catalytic efficiency toward complexes is presented. The relationship between structural features and catalytic efficiency toward
the oxidative functionalization of inexpensive and abundant raw‐materials to added‐value products the oxidative functionalization of inexpensive and abundant raw-materials to added-value products
is also addressed. is also addressed.
Keywords: C‐scorpionate; cyclic voltammetry; redox potential; catalyst; oxidation; electrochemical Keywords: C-scorpionate; cyclic voltammetry; redox potential; catalyst; oxidation; electrochemical
parameter; alkane; alkene; alcohol; ketone parameter; alkane; alkene; alcohol; ketone
1. Introduction 1. Introduction
Scorpionate compounds (Figure 1), in particular, poly(pyrazol‐1‐yl)borates, R1BXn(R2pz)3−n Scorpionate compounds (Figure 1), in particular, poly(pyrazol-1-yl)borates, R1 BXn (R2 pz)3−n
(pz = pyrazol‐1‐yl, n = 0 or 1), and poly(pyrazol‐1‐yl)methanes, R1CXn(R2pz)3−n (n = 0 or 1), are (pz = pyrazol-1-yl, n = 0 or 1), and poly(pyrazol-1-yl)methanes, R1 CXn (R2 pz)3−n (n = 0 or 1), are
undoubtedly among the most important N‐donor ligands in coordination chemistry [1–8]. The latter undoubtedly among the most important N-donor ligands in coordination chemistry [1–8]. The latter
are considerably less well studied than the analogous borate species [3]. However, in the last two are considerably less well studied than the analogous borate species [3]. However, in the last two
decades, mainly driven by improved syntheses [9,10], coordination behavior and physicochemical decades, mainly driven by improved syntheses [9,10], coordination behavior and physicochemical
properties of poly(pyrazol‐1‐yl)methanes have attracted considerable interest [5–8] in order to properties of poly(pyrazol-1-yl)methanes have attracted considerable interest [5–8] in order to perform
perform the precise tuning of target scorpionates towards a desired function [3,6,7]. Applications of the precise tuning of target scorpionates towards a desired function [3,6,7]. Applications of this highly
this highly versatile class of metal compounds range from organic synthesis, analytical, bio‐inorganic versatile class of metal compounds range from organic synthesis, analytical, bio-inorganic or catalytic
or catalytic chemistry to material sciences [3,6–8,11–18]. chemistry to material sciences [3,6–8,11–18].
Figure
1. General General
scorpionate
structure:
poly(pyrazol-1-yl)borates
for Z = B;
Figure 1. scorpionate structure: poly(pyrazol‐1‐yl)borates for Z = B; poly(pyrazol‐1‐
poly(pyrazol-1-yl)methanes
for
Z
=
C.
yl)methanes for Z = C. The development of sustainable efficient catalytic processes for the activation of abundant and The
development of sustainable efficient catalytic processes for the activation of abundant
inexpensive raw‐materials into high‐added‐value products remains a great challenge for both and inexpensive
raw-materials
into high-added-value
products
remains
a great
challenge
for
academic and industrial purposes. In this respect, the use of metal complexes bearing C‐scorpionate both
academic and industrial purposes. In this respect, the use of metal complexes bearing
poly(pyrazol‐1‐yl)methane ligands as catalysts is as
currently development C-scorpionate
poly(pyrazol-1-yl)methane
ligands
catalystsexperiencing is currently significant experiencing
significant development [6–8,16]. Transition metals are important in this topic participating e.g., in redox
Catalysts 2017, 7, 12; doi:10.3390/catal7010012 www.mdpi.com/journal/catalysts Catalysts 2017, 7, 12; doi:10.3390/catal7010012
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Catalysts 2017, 7, 12
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processes, which can be applied in specific steps. The electronic interaction between transition
metals and scorpionate ligands can play a key role in improving the redox process, and the type of
scorpionate ligand can be determinant in achieving the desired properties in such complexes. Thus,
one advantage of this catalytic system (over e.g., the metallocene based one) is the ease of modifying
the scorpionate ligand to change the steric and electronic properties of the metal complex and therefore,
its catalytic performance.
Industrially important reactions catalyzed by C-scorpionate complexes include [6–8,17–22]
(i) mild partial oxidation of alkanes to alkyl hydroperoxides, alcohols and ketones; (ii) epoxidation
of alkenes; (iii) oxidation of primary or secondary alcohols to aldehydes or ketones, respectively;
(iv) the Baeyer-Villiger oxidation of linear or cyclic ketones to the corresponding esters and lactones,
respectively; (v) the single pot carboxylation of gaseous alkanes into the corresponding Cn+1 carboxylic
acids; (vi) the stereo-selective nitroaldol Henry C–C coupling reaction; and (vii) olefin polymerization.
Whereas the main catalytic applications of metal complexes with tris(pyrazol-1-yl)borates
or heteroscorpionate ligands based on the bis(pyrazol-1-yl)methane moiety are found in olefin
polymerization reactions [1,6], tris(pyrazol-1-yl)methane-type complexes of several transition metals
are mainly used as catalysts or catalyst precursors for alkane, alkene, alcohol, and ketone oxidation
reactions directed toward single-pot organic synthesis [7]. Their use as catalysts for the C–C coupling
Henry reaction (a non-redox process) [8] has also proved to be a very promising strategy, in particular
for those metals (e.g., Zn) that exhibit no redox flexibility but can behave as Lewis acid catalysts.
Moreover, tris(pyrazol-1-yl)methane metal complexes can exhibit remarkable versatile catalytic
activity for oxidation reactions [16]. It is believed that the interchange between bidentate and tridentate
coordination modes of the C-scorpionate ligands is at the core of the structural and chemical versatility
of many metal complexes of this kind and is essential for their catalytic applications.
Electron transfer plays a fundamental role in governing the pathway of most of the above chemical
reactions. In fact, the activity of metal-based catalysts depends largely on their ligand environment and
coordination geometry, which also rule their oxidation/reduction properties, with the redox potential
as a determining parameter. Thus, quantification of the net electron donation of the ligands to a metal
center would allow predicting metal-centered redox potentials, and vice-versa, providing a powerful
tool for the design of metal-based catalysts within a desired redox window.
Determination of redox potentials can be conveniently done by e.g., the easy and fast cyclic
voltammetry technique, provided the redox signals lie within the available solvent/electrolyte potential
window and the species have a sufficient lifetime for signal detection. However, to date, the useful
information associated to the redox potential of a metal complex has not yet found a common
application as a characterization or identification tool [23]. Moreover, a survey of the redox properties
of known C-homoscorpionate metal complexes is missing.
Systematic approaches to establish redox potential/structure relationships, following the
recognition of full additive ligand (L) effects on that potential have been proposed [24–28]. For example,
Lever’s parametrization approach (Equation (1)) [27,28] allows for the prediction of an Mn+1/n redox
potential (E) of a six-coordinate metal complex in V vs. SHE (standard hydrogen electrode), where EL
is an additive ligand parameter obtained by a statistical analysis on the known redox potentials of a
high number of Mn+1/n complexes [27,28]. The slope, SM , and intercept, IM , are dependent upon the
metal and redox couple, the polygon of the complex, the spin state, and the stereochemistry [23].
E = SM (Σ EL ) + IM /V vs. SHE
(1)
On the other hand, Equation (1) can be applied to estimate the EL value of a ligand (L) provided
one knows the redox potential of a complex with that ligand L bound to a Mn+1/n metal redox couple
with known IM and SM parameters, and the EL values of the co-ligands.
Herein, the electrochemical properties of homoscorpionate tris(pyrazol-1-yl)methane metal
complexes that act as catalysts for the above industrial oxidation reactions are presented.
redox/catalytic activity relationships, a very important tool for the design of improved catalysts to address some of the problems presented by current large‐scale industrial partial oxidation processes. 2. C‐Homoscorpionates and Their Metal Catalysts Catalysts
2017, 7, 12
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Firstly reported by Trofimenko (1966) [29,30] as “a new and fertile field of remarkable scope”, B‐scorpionate tris(pyrazol‐1‐yl)borates (Tp, Figure 2a) indeed were revealed to be a class of compounds that became a precious ligand system in modern coordination chemistry [1,2]. lower
Moreover, the EL ligand parameter, a measure of the electron donor character of ligand L (the
Although discovered earlier by Hückel et al. (1937) [31], the analogous C‐scorpionate its value, the stronger that character), is used to establish redox/structure as well as redox/catalytic
tris(pyrazol‐1‐yl)methanes Figure remained dormant with respect to coordination activity
relationships, a very(Tpm, important
tool2b) for the
design of
improved
catalysts
to address
some of
chemistry until 1966 [32], mainly due to synthetic difficulties and usually very low yields associated the problems presented by current large-scale industrial partial oxidation processes.
with the preparation of functionalized tris(pyrazol‐1‐yl)methanes where substituents on the pyrazolyl rings are larger than methyl. In fact, until Elguero’s report (1984) of an improved synthetic 2.
C-Homoscorpionates and Their Metal Catalysts
strategy [33] and its subsequent application in the formation of functionalized derivatives bearing Firstly reported by Trofimenko (1966) [29,30] as “a new and fertile field of remarkable scope”,
bulky substituents [34,35], only few (less than 20) reports appeared pertaining to first‐row transition B-scorpionate tris(pyrazol-1-yl)borates (Tp, Figure 2a) indeed were revealed to be a class of compounds
metal complexes of tris(pyrazol‐1‐yl)methanes, mostly homoleptic ones. that became a precious ligand system in modern coordination chemistry [1,2].
Figure
2. Structural general representation of tris(pyrazol-1-yl)borate (a); tris(pyrazol-1-yl)methane (b);
Figure 2. Structural general representation of tris(pyrazol‐1‐yl)borate (a); tris(pyrazol‐1‐yl)methane and
cyclopentadienyl
(c) ligands.
(b); and cyclopentadienyl (c) ligands. It is commonly agreed earlier
to compare [2,3,30,36–38] main [31],
characteristics of tris(pyrazol‐1‐yl) Although
discovered
by Hückel
et al. the (1937)
the analogous
C-scorpionate
type scorpionate ligands with other face‐capping ligands. In particular, the parallel between Tp and tris(pyrazol-1-yl)methanes
(Tpm, Figure 2b) remained dormant with respect to coordination chemistry
cyclopendadienyl (Cp, Figure 2c) ligands is established in that both are mononegative, six‐electron until
1966 [32], mainly due to synthetic difficulties and usually very low yields associated with
(ionic model) or five‐electron donor (covalent model) ligands. They are also formally isolobal [2,3]. the
preparation of functionalized tris(pyrazol-1-yl)methanes where substituents on the pyrazolyl
The former are weak‐field hard σ‐N donors which tend to behave as fac‐capping chelating ligands rings
are larger than methyl. In fact, until Elguero’s report (1984) of an improved synthetic
(i.e., occupy three coordination positions), while Cp are typically 5‐fold π‐donors and tend to form strategy
[33] and its subsequent application in the formation of functionalized derivatives bearing
tetrahedral complexes [3,38]. few (less than 20) reports appeared pertaining to first-row transition
bulky
substituents [34,35], only
metalImportantly, it has been shown that there is no systematic trend in comparative electron donor complexes of tris(pyrazol-1-yl)methanes, mostly homoleptic ones.
ability of Tp relative to Cp [39]. Their electron‐donating abilities are dependent upon the identity and It is commonly agreed to compare [2,3,30,36–38] the main characteristics of tris(pyrazol-1-yl)
oxidation state of the metal center as well as the properties of the other ligands in the complex [40]. type
scorpionate ligands with other face-capping ligands. In particular, the parallel between Tp and
Tris(pyrazol‐1‐yl)borates are also bulkier isthan the formally Cp and, in six‐coordinate cyclopendadienyl
(Cp, Figure
2c) ligands
established
in thatanalogous both are mononegative,
six-electron
environments, enforce nearly octahedral coordination to the metal with N‐M‐N bite angles close to (ionic
model) or five-electron donor (covalent model) ligands. They are also formally isolobal [2,3].
the ideal value (90°). This hard
has been suggested to be the toprimary of the different reactivity The
former
are weak-field
σ-N donors
which
tend
behave source as fac-capping
chelating
ligands
between comparable Tp and Cp complexes [40,41]. For example, the greater steric Tp (i.e.,
occupy
three coordination
positions),
while
Cp are
typically
5-fold
π-donors
and profile tend toof form
ligands has permitted the isolation of molecular species whose pentamethylcyclopentadienyl tetrahedral
complexes
[3,38].
congeners proved ittoo [42]. that
In addition, Tp is coordinatively flexible, presenting κ2‐ or κ3‐
Importantly,
hasreactive been shown
there is no
systematic
trend in comparative
electron
donor
coordination modes to(i.e., the scorpionate feature). The carbon Tp, the
tris(pyrazol‐1‐
ability
of Tp relative
Cp [39].
Their electron-donating
abilities
areanalogues dependent to upon
identity and
yl)methanes, maintain the tripodal face capping aspect and the same electro‐donor ability, but differ oxidation
state of the metal center as well as the properties of the other ligands in the complex [40].
from Tp and Cp in the charge they hold (Figure 2). Tris(pyrazol-1-yl)borates
are also bulkier than the formally analogous Cp and, in six-coordinate
environments, enforce nearly octahedral coordination to the metal with N-M-N bite angles close to the
ideal value (90◦ ). This has been suggested to be the primary source of the different reactivity between
comparable Tp and Cp complexes [40,41]. For example, the greater steric profile of Tp ligands has
permitted the isolation of molecular species whose pentamethylcyclopentadienyl congeners proved
too reactive [42]. In addition, Tp is coordinatively flexible, presenting κ2 - or κ3 -coordination modes
(i.e., the scorpionate feature). The carbon analogues to Tp, tris(pyrazol-1-yl)methanes, maintain the
tripodal face capping aspect and the same electro-donor ability, but differ from Tp and Cp in the charge
they hold (Figure 2).
Catalysts 2017, 7, 12
Catalysts 2017, 7, 12 Catalysts 2017, 7, 12 4 of 21
4 of 21 4 of 21 Since 2005 a considerable interest in the development of a fast an efficient synthetic route for Since 2005 a considerable interest in the development of a fast an efficient synthetic route for Since
2005 a considerable interest in the development of a fast an efficient synthetic route for
hydrotris(pyrazol‐1‐yl)methane, HC(pz)
3 (pz = pyrazol‐1‐yl), Tpm [10], as well as on the design and hydrotris(pyrazol‐1‐yl)methane, HC(pz)
hydrotris(pyrazol-1-yl)methane,
HC(pz)33 (pz = pyrazol‐1‐yl), Tpm [10], as well as on the design and (pz = pyrazol-1-yl), Tpm [10], as well as on the design
1C(R2pz)3, overcoming the lack in synthesis of brand new poly‐functionalized C‐homoscorpionates, R
1C(R2pz)31, overcoming the lack in synthesis of brand new poly‐functionalized C‐homoscorpionates, R
and synthesis of brand new poly-functionalized C-homoscorpionates,
R C(R2 pz)3 , overcoming
the chemistry of such species (Figure 3), has been found [43–45]. For example, new tris(pyrazol‐1‐
the chemistry of such species (Figure 3), has been found [43–45]. For example, new tris(pyrazol‐1‐
the
lack in the chemistry of such species (Figure 3), has been found [43–45]. For example, new
yl)methanes functionalized at the methine carbon
atom (in order to vary the coordination behavior yl)methanes functionalized at the methine carbon
atom (in order to vary the coordination behavior tris(pyrazol-1-yl)methanes
functionalized at the methine
carbon atom (in order to vary the coordination
and physicochemical properties) were successfully prepared: CH33SO
3CH2C(pz)3 [43] or and physicochemical properties) were successfully prepared: 3CH
2C(pz)
3 [43] or behavior and physicochemical properties) were successfully
prepared:CH
CHSO
3 SO
3 CH
2 C(pz)
3 [43] or
PyCH
2OCH2C(pz)3 (Py = pyridine) [44]. The functionalization of pyrazol‐1‐yl rings (to modulate the PyCH22OCH
OCH22C(pz)
PyCH
C(pz)33 (Py = pyridine) [44]. The functionalization of pyrazol‐1‐yl rings (to modulate the (Py = pyridine) [44]. The functionalization of pyrazol-1-yl rings (to modulate the
coordination properties) was also achieved, as well as those derivatives that combine the two types coordination properties) was also achieved, as well as those derivatives that combine the two types coordination properties) was also achieved,
as well as those derivatives that combine the two types of
−
of functionalization: e.g., SO
3C(3‐Phpz)
−33− [45], HOCH
of functionalization: e.g., SO
3C(3‐Phpz)
[45], HOCH22C(3‐Phpz)
C(3‐Phpz)33 [44], or PyCH
[44], or PyCH22OCH
OCH22C(3‐Phpz)
C(3‐Phpz)33 [44]. [44]. functionalization:
e.g., SO3 C(3-Phpz)
3 [45], HOCH2 C(3-Phpz)3 [44], or PyCH2 OCH2 C(3-Phpz)3 [44].
Figure
3. Structures Structures offunctionalized functionalized tris(pyrazol-1-yl)methanes:(a) (a)
CH33SO
CH2 C(pz)
Figure 3; (b) 3;
Figure 3. 3. Structures of of functionalized tris(pyrazol‐1‐yl)methanes: tris(pyrazol‐1‐yl)methanes: (a) CH
CH33SO
SO3CH
CH322C(pz)
C(pz)
3; (b) − and
−
(b)
PyCH
OCH
C(pz)
(Py
=
pyridine)
or
PyCH
OCH
C(3-Phpz)
;
(c)
SO
C(3-Phpz)
2 2C(pz)
2 3 (Py = pyridine) or PyCH
3
2
2 3; (c) SO3C(3‐Phpz)
3
3
3 2C(3‐
2OCH2C(3‐Phpz)
3 and (d) HOCH
PyCH
OCH
2C(pz)3 (Py = pyridine) or PyCH2OCH2C(3‐Phpz)3; (c) SO3C(3‐Phpz)3− and (d) HOCH2C(3‐
PyCH22OCH
(d)
HOCH
2 C(3-Phpz)3 .
Phpz)
3. Phpz)3. A systematic investigation of the coordination behavior of new C‐scorpionates, as well as some A
systematic investigation of the coordination behavior of new C-scorpionates, as well as some
A systematic investigation of the coordination behavior of new C‐scorpionates, as well as some of the known ones, toward a variety of transition metals (e.g., V [44,46–49], Mo [17,19,50], Re [51,52], of
the
known
ones, toward a variety of transition metals (e.g., V [44,46–49], Mo [17,19,50], Re [51,52],
of the known ones, toward a variety of transition metals (e.g., V [44,46–49], Mo [17,19,50], Re [51,52], Fe [44,46,49,53], Ru [54], Co [12,13], Ni [44,49,55], Pd [44], Cu [43,45,46,56,57], Ag [14], Au [58] or Zn Fe
[44,46,49,53], Ru [54], Co [12,13], Ni [44,49,55], Pd [44], Cu [43,45,46,56,57], Ag [14], Au [58] or
Fe [44,46,49,53], Ru [54], Co [12,13], Ni [44,49,55], Pd [44], Cu [43,45,46,56,57], Ag [14], Au [58] or Zn [44,56]) followed, leading to new classes coordination Zn
[44,56])
followed,
leading
new
classesof ofcomplexes complexesexhibiting exhibitingdifferent different types types of of
[44,56]) followed, leading to to
new classes of complexes exhibiting different types of coordination
coordination modes (Figures 4 and 5). Like the pincer of a scorpion, these versatile tripodal ligands bind metal modes
(Figures
4
and
5).
Like
the
pincer
of
a
scorpion,
these
versatile
tripodal
ligands
bind metal
modes (Figures 4 and 5). Like the pincer of a scorpion, these versatile tripodal ligands bind metal centers with nitrogen atoms from two pyrazolyl rings attached to the central carbon atom; the third centers
with nitrogen atoms from two pyrazolyl rings attached to the central carbon atom; the third
centers with nitrogen atoms from two pyrazolyl rings attached to the central carbon atom; the third pyrazolyl attached to carbon rotates forward like a scorpionʹs tail to “sting” the metal; hence the name pyrazolyl
attached to carbon rotates forward like a scorpion’s tail to “sting” the metal; hence the name
pyrazolyl attached to carbon rotates forward like a scorpionʹs tail to “sting” the metal; hence the name of “scorpionates” (Figure 4). of
“scorpionates”
(Figure 4). of “scorpionates” (Figure 4). 2–κ3 interchange coordination modes of a tris(pyrazol‐1‐yl)methane scorpionate ligand and Figure 4. κ
2
3
Figure 4. κ
Figure
4. κ2–κ
–κ3 interchange coordination modes of a tris(pyrazol‐1‐yl)methane scorpionate ligand and interchange coordination modes of a tris(pyrazol-1-yl)methane scorpionate ligand and
comparison with a scorpion. comparison with a scorpion. comparison with a scorpion.
Catalysts 2017, 7, 12
Catalysts 2017, 7, 12 5 of 21
5 of 21 (a)
(b)
N
N
O
C
N
C
N
N N
N
N
M
N
N
N N
N
O
O
X
N
M'
O
Re
(BF4)2n
X
ReO4
O
N
N
N
P
N
N
n
N N
N
C
M = Fe(II)
M’ = Pd(II)
X = NO3, Cl
N
H
(c)
(d)
SO 3
N
C
C
N
N
N
N N
N
N
Ph
N
NN
Ag
PCy3
SO 2
O
Ph
Ph
N
Ag
L = PTA or PPh3 L
Figure 5. Selected C‐homoscorpionate complexes exhibiting: (a) tetradentate coordination ability of Figure
5. Selected C-homoscorpionate complexes exhibiting: (a) tetradentate coordination ability
2‐coordination of the scorpionate ligand at an octahedral geometry; (c) κ
2‐
the scorpionate ligand; (b) κ
of the scorpionate ligand; (b)
κ2 -coordination of the scorpionate ligand at an octahedral geometry;
2
3
‐ or N
2
O‐coordination of the coordination of the scorpionate ligand at a square planar geometry (d) N
(c) κ -coordination of the scorpionate ligand at a square planar geometry (d) N3 - or N2 O-coordination
ofscorpionate ligand. the scorpionate ligand.
Some of the above new C‐homoscorpionates (with three identical pyrazol‐1‐yl rings), such as Some of the above new C-homoscorpionates (with three identical pyrazol-1-yl rings), such as
e.g., PyCH2OCH2C(pz)3 (Figure 5a) exhibit extended coordination ability, including tetradentate e.g., PyCH2 OCH2 C(pz)3 (Figure 5a) exhibit extended coordination ability, including tetradentate
characteristics [44], where the extra coordination moiety has different affinity towards metal centers. characteristics [44], where the extra coordination moiety has different affinity towards metal centers.
Therefore, it leads to a sort of metal‐supported scorpionate ligand that forms easily heterobimetallic Therefore, it leads to a sort of metal-supported scorpionate ligand that forms easily heterobimetallic
species, opening to a large variety of applications (such as catalysis or supramolecular chemistry). species, opening to a large variety of applications (such as catalysis or supramolecular chemistry).
On the other hand, tris(pyrazol‐1‐yl)methane derivatives bearing bulky substituents at the On the other hand, tris(pyrazol-1-yl)methane derivatives bearing bulky substituents at the
pyrazol‐1‐yl rings (especially at the 3‐position), when ligating a metal center, such a bulky species pyrazol-1-yl rings (especially at the 3-position), when ligating a metal center, such a bulky species
provides a steric control on the other coordination position(s) of the complex, selecting the suitable provides a steric control on the other coordination position(s) of the complex, selecting the suitable
ligands on the opposite side, namely preventing the formation of full‐sandwich complexes (with two ligands on the opposite side, namely preventing the formation of full-sandwich complexes (with two
such scorpionate ligands) [45]. Moreover, they also offer the opportunity to tailor the coordination such
scorpionate
[45].
Moreover,
offer the
opportunity
to tailor
the coordination
behavior toward ligands)
different metal centers. they
This also
important feature is directly correlated to further behavior
toward
different
metal
centers.
This
important
feature
is
directly
correlated
to
further research
research in catalytic synthetic chemistry. in catalytic
synthetic chemistry.
It was also found, from electrochemical experiments [57,59], that changes on the functionalized It wasgroup also found,
from electrochemical
experiments
[57,59],
changes
on theon functionalized
methine of tris(pyrazol‐1‐yl) scorpionates have a much that
smaller influence the ligand methine
group
of
tris(pyrazol-1-yl)
scorpionates
have
a
much
smaller
influence
on
the
ligand
properties
properties than when performed at the pyrazol‐1‐yl rings (see below). than when
performed
at
the
pyrazol-1-yl
rings
(see
below).
The coordination versatility of tris(pyrazol‐1‐yl)methanes, namely the interchange between The
coordination versatility of tris(pyrazol-1-yl)methanes,
namely the interchange between
2‐coordination) was found to be tuned bidentate and tridentate coordination modes (Figure 5b,c for κ
2 -coordination) was found to be tuned by
bidentate
and
tridentate
coordination
modes
(Figure
5b,c
for
κ
by metal center as well as by the electronic properties of the co‐ligands present at the coordination metal
center as well as by the electronic properties of the co-ligands present at the
coordination
sphere.
sphere. In addition, the tripodal functionalized coordination flexibility (e.g. N
3‐ or N
2O‐coordination In
addition,
the
tripodal
functionalized
coordination
flexibility
(e.g.,
N
or
N
O-coordination
modes
3
2
modes for sulfonated derivatives, Figure 5d) involving the functionalized methine carbon for
sulfonated
derivatives,
Figure
5d)
involving
the
functionalized
methine
carbon
[7,8,14,45,49,50]
[7,8,14,45,49,50] is also tailored by such co‐ligands, an important pre‐requisite for their catalytic is
also
tailored by such co-ligands, an important pre-requisite for their catalytic activity.
activity. One
might consider that the coordination behavior of the tris(pyrazol-1-yl)methane complexes
One might consider that the coordination behavior of the tris(pyrazol‐1‐yl)methane complexes would
mirror
compounds,the themajor major
would mirror that
that exhibited
exhibited by
by the
the corresponding
corresponding tris(pyrazol-1-yl)borate
tris(pyrazol‐1‐yl)borate compounds, difference
However,large large
difference being
being in
in the
the charge
charge between
between the
the methane
methane and
and the
the borate
borate counterpart.
counterpart. However, differences appear in some cases [60]: for example, the RC(pz)3 ligands react with Group 6 metal Catalysts 2017, 7, 12
6 of 21
Catalysts 2017, 7, 12 6 of 21 differences
appear in some cases [60]: for example, the RC(pz)3 ligands react with Group
6 metal
Catalysts 2017, 7, 12 hexacarbonyls to afford insoluble and non‐volatile species, whereas [M{RB(pz)3}(CO)36 of 21 ] are very hexacarbonyls to afford insoluble and non-volatile species, whereas [M{RB(pz)3 }(CO)3 ] are very soluble
soluble and sublime easily. The tripodal ligand HC(pz)3 produces a relatively strong ligand field, to The
afford insoluble and HC(pz)
non‐volatile species, a whereas [M{RB(pz)
3}(CO)3] are very andhexacarbonyls sublime easily.
tripodal
ligand
relatively
strong ligand
field,
consistent
3 produces
consistent and with the rather short metal–nitrogen bond 3 lengths in the complexes. The pyrazol‐1‐yl easily. The tripodal HC(pz)
a relatively strong ligand field, acts as
withsoluble the rathersublime short metal–nitrogen
bondligand lengths
in theproduces complexes.
The pyrazol-1-yl
group
group acts as moderately strong σ donor and a weak out‐of‐plane π donor, with the π interaction in consistent with the rather short metal–nitrogen bond lengths with
in the The in
pyrazol‐1‐yl moderately
strong
σ donor
and
a weak
out-of-plane
π donor,
thecomplexes. π interaction
the plane of the
the plane of the amine ligand probably being close to zero [61]. group acts as moderately strong σ donor and a weak out‐of‐plane π donor, with the π interaction in amine ligand probably being close to zero [61].
the plane of the amine ligand probably being close to zero [61]. The main applications of C‐homoscorpionate complexes as catalysts for oxidative reactions, The main applications of C-homoscorpionate complexes as catalysts for oxidative reactions,
The main applications of C‐homoscorpionate complexes as catalysts for oxidative reactions, where the involvement of metal redox processes is crucial for the catalytic activity, are the important where
the involvement of metal redox processes is crucial for the catalytic activity, are the important
where the involvement of metal redox processes is crucial for the catalytic activity, are the important and challenging single‐pot oxidation of gaseous (e.g., direct oxidation of methane to carboxylic acids, andand challenging single‐pot oxidation of gaseous (e.g., direct oxidation of methane to carboxylic acids, challenging single-pot oxidation of gaseous (e.g., direct oxidation of methane to carboxylic
Scheme 1) [47,62]) and liquid (e.g., cyclohexane to the correspondent alcohol‐ketone mixture, Scheme acids,
Scheme 1) [47,62]) and liquid (e.g., cyclohexane to the correspondent alcohol-ketone mixture,
Scheme 1) [47,62]) and liquid (e.g., cyclohexane to the correspondent alcohol‐ketone mixture, Scheme 2) alkanes [7,8,13,43,46–49,53,57,58,63–68]. Indeed, oxidation of alkanes has been the object of Scheme
2) alkanes
[7,8,13,43,46–49,53,57,58,63–68].
of has alkanes
2) alkanes [7,8,13,43,46–49,53,57,58,63–68]. Indeed, Indeed,
oxidation oxidation
of alkanes been has
the been
object the
of object
considerable research [69–78], but still constitutes a serious challenge to modern chemistry, owing to considerable research [69–78], but still constitutes a serious challenge to modern chemistry, owing to of considerable
research [69–78], but still constitutes a serious challenge to modern chemistry, owing
the high inertness of these substrates. Currently, alkanes are mainly applied as fuels but it would be the high inertness of these substrates. Currently, alkanes are mainly applied as fuels but it would be to the
high inertness of these substrates. Currently, alkanes are mainly applied as fuels but it would
desirable to direct their application to the synthesis of organic products of a high added value. The desirable to direct their application to the synthesis of organic products of a high added value. The be desirable to direct their application to the synthesis of organic products of a high added value.
feasibility of this approach is supported by the industrial application of cyclohexane in the production Thefeasibility of this approach is supported by the industrial application of cyclohexane in the production feasibility of this approach is supported by the industrial application of cyclohexane in the
of cyclohexanone and cyclohexanol cyclohexanol (KA 2), 2), with dioxygen as oxidizing agent agent and and of cyclohexanone and (KA oil, oil, Scheme Scheme with dioxygen as oxidizing production of cyclohexanone and cyclohexanol (KA oil, Scheme 2), with dioxygen as oxidizing
catalysts based cobalt [79]. [79]. However, However, this process has has a very low low yield yield to ensure an catalysts based on on cobalt this industrial industrial process a very to ensure an agent
and catalysts based on cobalt [79]. However, this industrial process has a very low yield to
acceptable selectivity. Another case is the industrial production of acetic acid, a known commodity acceptable selectivity. Another case is the industrial production of acetic acid, a known commodity ensure
an acceptable selectivity. Another case is the industrial production
of acetic acid, a known
TM methanol carbonylation of large‐scale demand. Currently mainly obtained by the improved Cativa
TM methanol carbonylation of large‐scale demand. Currently mainly obtained by the improved Cativa
commodity
of large-scale
demand. requires Currently
mainly
by the
CativaTM
methanol
process [69,71,80], it nevertheless three steps obtained
from natural gas improved
and considerably harsh, process [69,71,80], it nevertheless requires three steps from natural gas and considerably harsh, carbonylation
process [69,71,80], it nevertheless requires three steps from natural gas and considerably
pollutant rich and costly conditions. The above examples explain the interest in finding more efficient pollutant rich and costly conditions. The above examples explain the interest in finding more efficient processes and in understanding the involved mechanisms [81–83]. harsh,
pollutant rich and costly conditions. The above examples explain the interest in finding more
processes and in understanding the involved mechanisms [81–83]. efficient processes and in understanding the involved mechanisms [81–83].
Scheme 1. One‐pot carboxylation of methane to acetic acid catalyzed by C‐scorpionate complexes [7,8,47,62]. Scheme
1. One-pot carboxylation of methane to acetic acid catalyzed by C-scorpionate complexes [7,8,47,62].
Scheme 1. One‐pot carboxylation of methane to acetic acid catalyzed by C‐scorpionate complexes [7,8,47,62]. Scheme
2. 2. Peroxidative
of cyclohexane cyclohexaneto to
cyclohexanone
cyclohexanol
(KA
Scheme Peroxidative oxidation
oxidation of cyclohexanone and and
cyclohexanol (KA oil) in oil) in
aqueous
medium, catalyzed by C-scorpionate catalysts [7,8,13,43,46–49,57–68].
aqueous medium, catalyzed by C‐scorpionate catalysts [7,8,13,43,46–49,57–68]. Scheme 2. Peroxidative oxidation of cyclohexane to cyclohexanone and cyclohexanol (KA oil) in Among the new catalysts recently found for the above reactions are C‐homoscorpionate metal Among
the new catalysts recently found for the above reactions are C-homoscorpionate metal
aqueous medium, catalyzed by C‐scorpionate catalysts [7,8,13,43,46–49,57–68]. complexes which have been successfully applied either as catalysts for oxygenations, with H
2O2, to complexes which have been successfully applied either as catalysts for oxygenations, with H2 O2 , to
produce the respective alcohols and ketones or, with K2S2O8, to directly yield carboxylic acids [7,8]. produce
the respective alcohols and ketones or, with K2 S2 O8 , to directly yield carboxylic acids [7,8].
Among the new catalysts recently found for the above reactions are C‐homoscorpionate metal The reactions leading to the above oxygenated species are believed to proceed mainly via both The reactions leading to the above oxygenated species are believed to proceed mainly via
both
carbon‐ and oxygen‐centered radicals. Interestingly, although occurring via the formation of reactive complexes which have been successfully applied either as catalysts for oxygenations, with H
2O2, to carbonand
oxygen-centered
radicals.
Interestingly,
although
occurring
via
the
formation
of
reactive
radicals, such reactions are rather selective [7,8,16]. The C‐scorpionate catalyst initially activates not produce the respective alcohols and ketones or, with K
2S2O8, to directly yield carboxylic acids [7,8]. radicals,
such
reactions
are
rather
selective
[7,8,16].
The
C-scorpionate
catalyst initially activates not the
the alkane but reacts with another reactant, usually the oxidant (e.g., hydrogen‐peroxide) [7,8]. The The reactions leading to the above oxygenated species are believed to proceed mainly via both formed reactive species (e.g., hydroxyl radical) attacks the alkane molecule without any participation alkane
but
reacts
with
another
reactant,
usually
the
oxidant
(e.g.,
hydrogen-peroxide)
[7,8]. The formed
carbon‐ and oxygen‐centered radicals. Interestingly, although occurring via the formation of reactive in the latter process of the metal complex. Thus, the metal catalyst does not take part in the direct reactive
species
(e.g.,
hydroxyl
radical)
attacks
the
alkane
molecule
without
any
participation
in the
radicals, such reactions are rather selective [7,8,16]. The C‐scorpionate catalyst initially activates not “activation” of the carbon‐hydrogen bond by the radical. latter
process of the metal complex. Thus, the metal catalyst does not take part in the direct “activation”
the alkane but reacts with another reactant, usually the oxidant (e.g., hydrogen‐peroxide) [7,8]. The A possible mechanism [69,70] for the generation of carboxylic acids is represented in Scheme 3 of
the
carbon-hydrogen
bond by the radical.
formed reactive species (e.g., hydroxyl radical) attacks the alkane molecule without any participation for an oxo‐V complex: a potassium peroxodisulfate salt (K2S2O8) is essential for the formation of alkyl A
possible
mechanism
[69,70] for the generation of carboxylic acids is represented in Scheme 3 for
in the latter process of the metal complex. Thus, the metal catalyst does not take part in the direct an
oxo-V complex: a potassium peroxodisulfate salt (K2 S2 O8 ) is essential for the formation of alkyl
“activation” of the carbon‐hydrogen bond by the radical. A possible mechanism [69,70] for the generation of carboxylic acids is represented in Scheme 3 for an oxo‐V complex: a potassium peroxodisulfate salt (K2S2O8) is essential for the formation of alkyl Catalysts 2017, 7, 12
Catalysts 2017, 7, 12 7 of 21
7 of 21 radicals whereas the scorpionate catalyst is not needed for this purpose. Nevertheless, no carboxylic radicals whereas the scorpionate catalyst is not needed for this purpose. Nevertheless, no carboxylic
acid is detected in the absence of the catalyst. Under the experimental conditions, peroxodisulfate acid is detected in the absence of the catalyst. Under the experimental conditions, peroxodisulfate
undergoes thermolysis into sulfate (or its protonated form HSO4•• if in acidic medium) radicals which undergoes thermolysis into sulfate (or its protonated form HSO4 if in acidic medium) radicals which
are known alkane hydrogen abstractors, leading • to R•. Further conversion of alkyl radical to are known alkane hydrogen abstractors, leading to R . Further conversion of alkyl radical to
carboxylic
•. The latter carboxylic acid includes carbonylation of the former by CO to form the acyl radical RCO
acid includes carbonylation of the former by CO to form the acyl radical RCO• . The latter may then
may then be converted, in the presence of the metal catalyst, by its oxygenation to •give RCOO•, be converted, in the presence of the metal catalyst, by its oxygenation to give RCOO , involving
a
involving a peroxo metal species (Scheme 3) derived from the reaction of catalyst with HS
2O8− or with − or with H SO
peroxo metal species (Scheme 3) derived from the reaction of catalyst
with
HS
O
2 8
2
5
H2SO5 (peroxomonosulfuric acid) formed upon reaction of HS
2O8− with TFA or hydrolysis by traces (peroxomonosulfuric acid)
formed upon reaction of HS2 O8 − with TFA or hydrolysis by traces of water.
•
of water. Then RCOOC
abstracts a hydrogen atom from, for example, excess TFA or alkane to afford Then RCOOC• abstracts a hydrogen atom from, for example, excess TFA or alkane to afford the desired
the desired carboxylic acid. carboxylic acid.
Scheme 3. Proposed mechanism for the carboxylation of an alkane to the corresponding carboxylic Scheme 3. Proposed mechanism for the carboxylation of an alkane to the corresponding carboxylic
acid catalyzed by a C‐homoscorpionate oxo‐V complex. acid catalyzed by a C-homoscorpionate oxo-V complex.
In the case
case ofof the oxidation alkanes with hydrogen peroxide, detailed investigation of the In the
the
oxidation
of of alkanes
with
hydrogen
peroxide,
detailed
investigation
of the effects
effects of various experimental parameters in this reaction, the use of radical traps, kinetic and of various experimental parameters in this reaction, the use of radical traps, kinetic and selectivity
selectivity studies complemented with theoretical [73,84–86]
calculations [73,84–86] indicated the interest of studies complemented
with theoretical
calculations
indicated
the interest
of using
controlled
•) and using controlled amounts of water and acid, and assured the involvement of hydroxyl (OH
•
•
amounts of water and acid, and assured the involvement of hydroxyl (OH ) and alkyl (R ) radicals
alkyl (R•) radicals in a radical type mechanism (see and
Equations and Scheme for a more in a radical
type mechanism
(see
Equations
(2)–(10)
Scheme (2)–(10) 4 for a more
detailed 4 formation
of
detailed formation of hydroxyl and hydroperoxyl radicals). hydroxyl and hydroperoxyl radicals).
The proposed route for the metal‐catalyzed decomposition of hydrogen peroxide (Haber–Weiss The proposed route for the metal-catalyzed decomposition of hydrogen peroxide (Haber–Weiss
mechanism) [70,87] includes the following two key stages (Equations (2) and (3)) of formation of the mechanism) [70,87] includes the following two key stages (Equations (2) and (3)) of formation of the
•
•
oxygen‐centered radicals HOO
oxygen-centered radicals HOO• and HO
and HO•: :
Mn+ + H2O2 → HO• + M(n+1)+ + HO− Mn+ + H2 O2 → HO• + M(n+1)+ + HO−
M(n+1)+ + H2O2 → HOO• + H+ + Mn+ (n+1)+
M
+ H2 O2 → HOO• + H+ + Mn+
(2)
(2)
(3)
(3)
Catalysts 2017, 7, 12
Catalysts 2017, 7, 12 8 of 21
8 of 21 Scheme 4. Proposed mechanism for the formation of OH
Scheme
4. Proposed mechanism for the formation of OH•• and HOO
and HOO•• radicals in the oxidation of an radicals in the oxidation of an
alkane
with hydrogen peroxide, catalyzed by a C-homoscorpionate oxo-V complex.
alkane with hydrogen peroxide, catalyzed by a C‐homoscorpionate oxo‐V complex. O2 by the reduced form of the metal ItIt is the hydroxyl radical (derived from the reduction of H
is the hydroxyl radical (derived from the reduction of H22O
2 by the reduced form of the metal
catalyst, Equation (2) and Scheme 4 for a C‐homoscorpionate oxo‐V complex) that reacts with the catalyst, Equation (2) and Scheme 4 for a C-homoscorpionate oxo-V complex) that reacts with the
alkane generating
generating the
the alkyl radical R• (Equation (4)) which, turn, reacts with (Equation
dioxygen alkane
alkyl
radical
R• (Equation
(4)) which,
in turn, in reacts
with
dioxygen
•. The latter gives rise to the alkyl‐hydroperoxide (Equation (5)) to form the alkylperoxyl radical ROO
•
(5)) to form the alkylperoxyl radical ROO . The latter gives rise to the alkyl-hydroperoxide (ROOH)
(ROOH) (Equation (6)) which, in the presence of both the reduced and oxidized forms of the metal (Equation
(6)) which, in the presence of both the reduced and oxidized forms of the metal catalyst,
catalyst, decomposes (Equations (7)–(10)) to the ketone and/or the alcohol. decomposes (Equations (7)–(10)) to the ketone and/or the alcohol.
HO• + RH → H2O + R• HO• + RH → H2 O + R•
R•+ O2 → ROO• R• +2O
O22 → ROOH + HOO
→ ROO•
• ROO• + H
(4)
(4)
(5)
(5)
(6)
•
•
ROO
+ H2n+
O → RO
+− + M
HOO
• + HO
(n+1)+ 2 → ROOH
ROOH + M
(6)
(7)
•
−
(n+1)+
ROOH
+ Mn+ (n+1)+
→ RO
+ HO
++M
• + H
ROOH + M
→ ROO
+ Mn+ (7)
(8)
ROOH +RO
M(n+1)+
→ ROO• + H+• + Mn+
• + RH → ROH + R
(8)
(9)
RO•• → ROH + R
+ RH → ROH
+ R• 2 2ROO
‐H = O + O
•
(9)
(10)
2ROO
→ ROHto + catalyze R-H = O +epoxidation O2
(10)
The use of C‐homoscorpionate complexes of alkenes, a very useful synthetic transformation to produce fine chemicals [79,80], is also significant. Selective catalytic The use of C-homoscorpionate complexes to catalyze epoxidation of alkenes, a very
epoxidation of cis‐cyclooctene to 1,2‐epoxy‐cyclooctane was achieved in the presence of tris(pyrazol‐
useful synthetic transformation to produce fine chemicals [79,80], is also significant. Selective
1‐yl)methane Mo(VI) complexes [19,20,88], in particular if water is rigorously excluded from the catalytic epoxidation of cis-cyclooctene to 1,2-epoxy-cyclooctane was achieved in the presence
reaction mixture. Other alkene substrates such as R‐(+)‐limonene, 1‐octene, trans‐2‐octene, of tris(pyrazol-1-yl)methane Mo(VI) complexes [19,20,88], in particular if water is rigorously
cyclododecene, 3‐carene, and 4‐vinyl‐1‐cyclohexene are also selectively converted into the excluded from the reaction mixture. Other alkene substrates such as R-(+)-limonene, 1-octene,
corresponding epoxides. trans-2-octene, cyclododecene, 3-carene, and 4-vinyl-1-cyclohexene are also selectively converted
In the presence of the sacrificial oxidant PhI(OAc)2, aqua Ru(II) tris(pyrazol‐1‐yl)methane into the corresponding epoxides.
compounds catalyze the aerobic epoxidation of a wide variety of alkenes [89]. The mechanistic pathway for the epoxidation (Scheme 5) proceeds via the formation of an active metal–oxo intermediate through the mediation of iodobenzene diacetate. The active catalytic species is a Catalysts 2017, 7, 12
9 of 21
In the presence of the sacrificial oxidant PhI(OAc)2 , aqua Ru(II) tris(pyrazol-1-yl)methane
compounds catalyze the aerobic epoxidation of a wide variety of alkenes [89]. The mechanistic pathway
for
the epoxidation (Scheme 5) proceeds via the formation of an active metal–oxo intermediate through
Catalysts 2017, 7, 12 9 of 21 the mediation of iodobenzene diacetate. The active catalytic species is a formally Ru(IV)=O one,
resulting
from the oxidation of the aqua Ru(II) complex by PhI(OAc)2 . The electrophilic metal bound
formally Ru(IV)=O one, resulting from the oxidation of the aqua Ru(II) complex by PhI(OAc)
2. The oxo
group subsequently interacts with the incoming olefinic double bond with transfer of the oxo
electrophilic metal bound oxo group subsequently interacts with the incoming olefinic double bond group.
The involvement of a concerted transition state for the transfer of the oxygen atom from the
with transfer of the oxo group. The involvement of a concerted transition state for the transfer of the metal-oxido
complex to the olefinic double bond is suggested [89,90].
oxygen atom from the metal‐oxido complex to the olefinic double bond is suggested [89,90]. Scheme 5. Proposed mechanism for the epoxidation of alkenes catalyzed by a C‐homoscorpionate Scheme
5. Proposed mechanism for the epoxidation of alkenes catalyzed by a C-homoscorpionate
aqua‐Ru complex. aqua-Ru
complex.
From the above, it turns out that an effective catalyst for the above oxidation reactions requires From the above, it turns out that an effective catalyst for the above oxidation reactions requires the
the ability to undergo reversible redox processes involving electron transfer at accessible potentials. ability to undergo reversible redox processes involving electron transfer at accessible potentials.
This redox potential—oxidative catalytic activity relationship will be addressed in detail in the This redox potential—oxidative catalytic activity relationship will be addressed in detail in the
following section. following section.
3. Electrochemical Properties of C‐Scorpionate Metal Complexes 3. Electrochemical Properties of C-Scorpionate Metal Complexes
The electrochemical approach is a very powerful tool for fundamental chemical characterization The electrochemical approach is a very powerful tool for fundamental chemical characterization of
of species that can be oxidized or reduced. By continuously changing the working potential, its species that can be oxidized or reduced. By continuously changing the working potential, its cycling or
cycling or keeping constant, enables not only the determination of the respective oxidation or keeping constant, enables not only the determination of the respective oxidation or reduction potentials
reduction potentials but also revelation of the reversibility of the redox processes, the nature, kinetics but also revelation of the reversibility of the redox processes, the nature, kinetics and equilibrium
and equilibrium constants of the follow‐up reactions, the stability and structure of intermediates, the constants of the follow-up reactions, the stability and structure of intermediates, the type and yield
type and yield of products, etc. In fact, an electron transfer in a coordination compound can induce of products, etc. In fact, an electron transfer in a coordination compound can induce very diverse
very diverse chemical reactivity, ultimately with catalytic significance. chemical reactivity, ultimately with catalytic significance.
Some C‐homoscorpionate complexes underwent systematic electrochemical investigation Some C-homoscorpionate complexes underwent systematic electrochemical investigation usually
usually by cyclic voltammetry (CV) and controlled potential electrolysis (CPE) techniques, at by cyclic voltammetry (CV) and controlled potential electrolysis (CPE) techniques, at platinum
platinum working electrodes (disk or gauze, respectively). Glassy carbon working electrodes for CV working electrodes (disk or gauze, respectively). Glassy carbon working electrodes for CV were
were also used [89]. Experiments were performed in a three‐electrode system whose potential was also used [89]. Experiments were performed in a three-electrode system whose potential was
controlled vs. a Luggin capillary connected to a silver wire pseudo‐reference electrode and a Pt auxiliary electrode. The complexes were added to a 0.1–0.2 M nBu4N[X] (X = BF4, PF6 or ClO4) or [Et4N][ClO4]/aprotic non‐aqueous medium (e.g., CH2Cl2, NCMe, DMF or DMSO), at room temperature, under dinitrogen [46,49–54,89,91–93]. Their measured redox potentials in volts vs. saturated calomel electrode (V vs. SCE) and the eventual reversibility of the redox process, are indicated in Table 1. Catalysts 2017, 7, 12
10 of 21
controlled vs. a Luggin capillary connected to a silver wire pseudo-reference electrode and a Pt
auxiliary electrode. The complexes were added to a 0.1–0.2 M [n Bu4 N][X] (X = BF4 , PF6 or ClO4 )
or [Et4 N][ClO4 ]/aprotic non-aqueous medium (e.g., CH2 Cl2 , NCMe, DMF or DMSO), at room
temperature, under dinitrogen [46,49–54,89,91–93].
Their measured redox potentials in volts vs. saturated calomel electrode (V vs. SCE) and the
eventual reversibility of the redox process, are indicated in Table 1.
Table 1. Cyclic voltammetric data a for metal C-homoscorpionate complexes.
Redox Potential/V vs. SCE
C-Scorpionate Compound
Ref.
I E ox (I E ox )
p
1/2
I E red (I E red )
p
1/2
II E red (II E red)
p
1/2
[VCl3 {κ3 -SO3 C(pz)3 }]
[VO2 {κ3 -SO3 C(pz)3 }] b
[VOCl2 {κ3 -CH3 SO2 OCH2 C(pz)3 }] c
[VO2 {κ3 -HC(pz)3 }][BF4 ] b
[VO2 {κ3 -HC(3,5-Me2 pz)3 }][BF4 ]
(1.14)
(1.35)
-
−0.46
−0.78
−0.28
−0.37
−1.82
−1.70
−1.75
[91]
[47]
[49]
[47]
[47]
Li[Mo{κ3 -SO3 C(pz)3 }(CO)3 ]
[Mo{κ3 -SO3 C(pz)3 }I(CO)3 ]
[Mo{κ3 -SO3 C(pz)3 }H(CO)3 ]
(0.18)
0.44
0.09
-
-
[50]
[50]
[50]
[ReCl2 {κ3 -HC(pz)3 }(PPh3 )][BF4 ] d
[ReCl3 {κ3 -HC(pz)3 }]
[ReCl3 {κ3 -HC(3,5-Me2 pz)3 }]
[ReCl4 {κ2 -HC(pz)3 }]
[ReO3 {κ3 -SO3 C(pz)3 }]
[ReO{κ3 -SO3 C(pz)3 }(HMT)] b
[ReOCl{κ3 -SO3 C(pz)3 }(PPh3 )]Cl
[ReO3 {κ2 -HC(pz)3 }(PTA)][ReO4 ] b
[ReO3 (Hpz)(HMT)][ReO4 ] b
(0.54)
1.14
(1.25)
1.79
(0.86)
1.45
-
(−0.74)
−0.62
(−0.13)
(−0.06)
−0.83
−0.83
(−0.94)
(−0.62)
(−0.33)
−1.70
(−0.72)
−1.50
(−1.41)
-
[51]
[52]
[52]
[52]
[52]
[21]
[52]
[21]
[21]
[FeCl2 {κ3 -CH3 SO2 OCH2 C(pz)3 }] c
[FeCl3 {κ3 -HC(pz)3 }] d
[FeCl3 {κ3 -HC(3,5-Me2 pz)3 }] d
[FeCl3 {κ3 -HC(3-iPrpz)3 }] d
(1.06)
(−0.11)
(−0.20)
(−0.04)
−0.38
-
-
[49]
[92]
[92]
[92]
[Ru(p-cymene){κ3 -SO3 C(pz)3 }]Cl
[Ru(p-cymene){κ3 -SO3 C(pz)3 }][BF4 ]
[Ru(p-cymene){κ3 -SO3 C(3-Phpz)3 }]Cl
[Ru(benzene){κ3 -SO3 C(pz)3 }]Cl
[Ru(benzene){κ3 -SO3 C(3-Phpz)3 }]Cl
[Ru(HMB){κ3 -SO3 C(pz)3 }]Cl
[Ru(cod)Cl{κ3 -SO3 C(pz)3 }]
[Ru(cod)Cl{κ3 -SO3 C(3-Phpz)3 }]
[RuCl{κ3 -HC(pz)3 }(bqdi)][ClO4 ] c
[Ru(H2 O){κ3 -HC(pz)3 }(bqdi)][ClO4 ]2 c
[Ru{κ3 -HC(3,5-Me2 pz)3 }(NCCH3 )3 ][BF4 ]2 c
[Ru{κ3 -HC(3,5-Ph2 pz)3 }(NCCH3 )3 ][BF4 ]2 c
(0.95)
(0.96)
1.02
(1.07)
(1.37)
0.95
(0.96)
(0.99)
(0.82)
(0.44)
(0.42)
(0.71)
(−0.97)
(−0.97)
(−1.00)
(−0.87)
(−0.92)
(−1.11)
(−1.10)
(−1.27)
(−0.79)
-
−1.39
-
[54]
[54]
[54]
[54]
[54]
[54]
[54]
[54]
[89]
[89]
[93]
[93]
[Co(OSO3 H)(OCH3 )(HOCH3 ){κ3 -HC(pz)3 }] b
[Co{κ3 -HOCH2 C(pz)3 }2 ](NO3 )2
[Co{κ3 -HOCH2 C(pz)3 }2 ]·[Co{κ3 -HOCH2 C(pz)3 }
(H2 O)3 ]2 (Cl)6 ·6H2 O
[CoCl2 (H2 O){κ3 -PyCH2 OCH2 C(pz)3 }]
[CoCl2 (H2 O){κ3 -CH3 SO2 OCH2 C(pz)3 }] c
1.03
(0.58)
−0.40
−0.68
(0.60)
−0.67
−1.21
[12]
1.28
1.10
−0.60
−0.64
-
[12]
[12]
[CuCl2 {κ3 -CH3 SO2 OCH2 C(pz)3 }] c
-
−0.70
-
[49]
[AuCl2 {κ2 -HC(pz)3 }]Cl c
[AuCl2 {κ2 -HOCH2 C(pz)3 }]Cl c
[AuCl2 {κ2 -HC(3,5-Me2 pz)3 }]Cl c
-
−0.02
−0.01
−0.11
−0.60
−0.58
−0.69
[57]
[57]
[57]
[12]
[12]
Values in V ± 0.02 relative to SCE; in CH2 Cl2 ; scan rate of 200 mV·s−1 . Values for reversible waves are
given in brackets. bqdi = o-benzoquinonediimine; 3-iPr = iso-propyl group; b In dimethyl sulfoxide (DMSO);
c In acetonitrile (NCMe); d In dimethylformamide (DMF). SCE = saturated calomel electrode.
a
Catalysts 2017, 7, 12
11 of 21
All authors found that C-homoscorpionate ligands are electrochemically inert in the potential
range of −2.0 V to 2.0 V vs. SCE, at the used experimental conditions [46,49–54,89,91–93], thus no
ligand centered oxidation or reduction has been reported to date.
Most of the metallic compounds bearing tris(pyrazol-1-yl)methane ligands exhibit at least a
single-electron (determined by exhaustive CPE) oxidation wave, assigned to the dn → dn −1 metal
oxidation. Exceptions are, as expected, V(V), Re(VII), Ni(II), Cu(II), Au(III), and Zn(II) complexes.
The said oxidation waves can meet the reversibility criteria [94] or be irreversible due to chemical
reactions that follow the electron-transfer process (Table 1). Most of the C-homoscorpionate complexes
also exhibit (Table 1) a reduction wave which usually is followed, at a lower potential, by a second one.
These waves often (e.g., for V, Re, Fe, Ru or Co complexes) correspond to single-electron processes,
being assigned to the dn → dn+1 and dn+1 → dn+2 metal reductions.
The highest known first oxidation potential of all C-scorpionate metal complexes is shown by
the 15-electron Re(IV) complex [ReCl4 {κ2 -HC(pz)3 }] (I Ep ox = 1.79 V vs. SCE, Table 1) per its electron
deficiency. Such oxidation potential value is even higher than the one of the oxo-Re(V) 16-electron
complex [ReOCl{κ3 -SO3 C(pz)3 }(PPh3 )]Cl (I Ep ox = 1.45 vs. SCE, Table 1) in spite of the higher metal
oxidation state of the latter. The presence of the strong electron-donor oxo-ligand provides another
reason for the lower oxidation potentials of this oxo-complex. [ReCl4 {κ2 -HC(pz)3 }] is also the one
that exhibits the most favorable (highest) reduction potential (I E1/2 ox = −0.06 V vs. SCE, Table 1) in
accord with its low electron-count. Harder to reduce are the oxo-Re species [ReO3 {κ3 -SO3 C(pz)3 }]
and [ReOCl{κ3 -SO3 C(pz)3 }(PPh3 )]Cl, in agreement with the presence of the strong electron-donor
oxide ligand and with their higher electron count. Among the rhenium complexes, Re(III) 16-electron
complexes [ReCl3 {κ3 -HC(pz)3 }] and [ReCl3 {κ3 -HC(3,5-Me2 pz)3 }] are those that present the lowest
oxidation potential (I Ep ox = 1.14 and I E1/2 ox = 1.25 V vs. SCE, Table 1), consistent with the lower metal
oxidation state. In contrast with the measured values, complex [ReCl3 {κ3 -HC(pz)3 }] would be expected
to have a higher oxidation potential than the analogous [ReCl3 {κ3 -HC(3,5-Me2 pz)3 }], on account of the
weaker electron-donor character of HC(pz)3 in the former in comparison with HC(3,5-Me2 pz)3 in the
latter. However, the irreversible character of the oxidation wave of the former (indicative of a chemical
reaction following the electron-transfer step, with a resulting shift of the oxidation potential) preclude
a reliable comparison between the measured potentials for these complexes.
In the case of Mo(0 or II) complexes a second single-electron oxidation process is detected (not
shown in Table 1) in the potential range of 0.18 to 0.6 V vs. SCE. In Li[Mo{κ3 -SO3 C(pz)3 }(CO)3 ] yields
the 16-electron Mo(II) complex [Mo{κ3 -SO3 C(pz)3 }(CO)3 ]+ , its irreversibility being associated to fast
coordination of a solvent molecule, leading to an electronically saturated product.
The irreversibility of the first oxidation wave of compounds [MoI{κ3 -SO3 C(pz)3 }(CO)3 ] and
[MoH{κ3 -SO3 C(pz)3 }(CO)3 ] signals the instability of the resulting cationic Mo(III) complexes,
which then rapidly decompose with probable CO loss [95] and, for the hydride compound, by
deprotonation [96–98]. The first oxidation potentials of all these tricarbonyl complexes are much lower
than that of the parent hexacarbonyl compound, on account of the replacement of three carbonyls in
the latter by the more electron-donating C-scorpionate ligands [24,27,28]. Moreover, the first oxidation
potential of [MoH{κ3 -SO3 C(pz)3 }(CO)3 ] in comparison with [MoI{κ3 -SO3 C(pz)3 }(CO)3 ] reflects the
stronger electron-donor character of the hydride relatively to the iodide ligand [28].
These Mo (0 or II) compounds have not yet been used for catalytic oxidation reactions.
Nevertheless, their low oxidation potentials (first oxidation wave in the range 0.09–0.44 V vs. SCE,
Table 1) and the detected easy coordination/decoordination of substrates are promising features for a
possible good oxidative catalytic performance.
The interest in electron transfer induced reactivity of C-scorpionate metal compounds
is demonstrated in the following catalytic systems where such complexes provide
unprecedented examples.
Catalysts 2017, 7, 12
12 of 21
3.1. Oxidation of Alkanes to Alcohols and Ketones
In the case of the oxidation of alkanes with peroxides, the availability of reducible metal
species, easily detectable by electrochemical experiments, was found very important for the catalytic
performance of C-homoscorpionate complexes.
As previously mentioned, the formation of RO• and ROO• radicals (Equations (2) and (3), and
Scheme 4) involves the reaction of both reduced and oxidized forms of the metal catalyst and is a
key step for the occurrence of the C–H abstraction from the alkane. Therefore, C-homoscorpionate
complexes that undergo redox processes at accessible potential values are expected to display better
oxidative catalytic performance than those harder to oxidize or reduce.
In fact, V(V) complexes [VO2 {κ3 -SO3 C(pz)3 }] and [VO2 {κ3 -HC(pz)3 }][BF4 ], whose accessible
potential values for the first single-electron [V(V) to V(IV)] reduction process are −0.46 and −0.48 V
vs. SCE, lead to quite similar (19% and 18.6%, respectively [47]) KA oil yields (among the highest
values obtained for this class of catalysts) by catalytic oxidation of cyclohexane. The turnover number
(TON, moles of product per mole of catalyst) values also follow the trend: 117 and 112, respectively,
for [VO2 {κ3 -SO3 C(pz)3 }] and [VO2 {κ3 -HC(pz)3 }][BF4 ].
A further example comes from V(III or IV) complexes. [VCl3 {κ3 -SO3 C(pz)3 }] is easier to oxidize
than [VOCl2 {κ3 -CH3 SO2 OCH2 C(pz)3 }] (1.14 vs. 1.35 V, Table 1) and thus yields higher KA oil amounts
(13% (TON = 121) [46] vs. 7% (TON = 89) in the presence of [VOCl2 {κ3 -CH3 SO2 OCH2 C(pz)3 }] [49]).
Moreover, trichlorovanadium(III) [VCl3 {κ3 -HC(pz)3 }] leads to higher yield (18%) and TON (167)
values [47] than the related [VCl3 {κ3 -SO3 C(pz)3 }] (13% yield and a TON of 121, [46,47]), in accordance
with its lower oxidation state and the neutral scorpionate ligand in[VCl3 {κ3 -HC(pz)3 }].
Likewise, for the Co(II) complexes [CoCl2 (H2 O){κ3 -PyCH2 OCH2 C(pz)3 }] and [CoCl2 (H2 O){κ3 CH3 SO2 OCH2 C(pz)3 }]; the latter presents lower oxidation potential (1.10 V vs. SCE, Table 1) and
thus exhibits better catalytic performance: 10.5% vs. 3.2% yield of KA oil in the presence of
[CoCl2 (H2 O){κ3 -PyCH2 OCH2 C(pz)3 }] [12] which is oxidized at 1.28 V vs. SCE (Table 1).
A similar behavior is found for the chloro-Au(III) complexes: [AuCl2 {κ2 -HC(pz)3 }]Cl and
[AuCl2 {κ2 -HOCH2 C(pz)3 }]Cl which present very close reduction potentials for the first irreversible
two electrons Au(III) → Au(I) reduction process (Table 1) and are the most active, leading to 8.1%
and 10.3%, respectively, of KA oil [57]. The hardest to reduce (−0.11 V vs. SCE, Table 1) yields only
7.5% of the oxygenated mixture under the same conditions [64]. The lower reduction potential of
[AuCl2 {κ2 -HC(3,5-Me2 pz)3 }]Cl in comparison with the one of [AuCl2 {κ2 -HC(pz)3 }]Cl is consistent
with the stronger electron-donor ability of the methyl-substituted κ2 -HC(3,5-Me2 pz)3 ligand than that
of κ2 -HC(pz)3 [24]. However, an accurate comparison cannot be established due to the irreversibility
of the reduction waves (the reduction potential is not the thermodynamic one). Moreover, whereas
the CH2 OH substituent at the apical methine carbon appears to have limited influence on the redox
potential of the gold complexes (Table 1), the replacement of hydrogens by an electron donor group
(Me) at the pyrazolyl rings of the C-scorpionate leads to an electronically richer Au(III) center, resulting
in a measurable (ca. 0.1 V) cathodic shift of the potential. A second irreversible reduction (Table 1)
assigned to the Au(I) → Au(0) reduction leads to the appearance of gold metal at the platinum
electrodes surface after exhaustive controlled potential electrolysis and an irreversible anodic wave
(in the range 0.44–0.50 V vs. SCE) observed upon scan reversal after the second reduction wave,
corresponding to the oxidation of the Au(0) species formed in the second reduction process.
3.2. Oxidation of Alkanes to Carboxylic Acids
The catalytic activity of the only to date tested [51] Re(III) complexes, [ReCl3 {κ3 -HC(pz)3 }] and
[ReCl2 {κ3 -HC(pz)3 }(PPh3 )][BF4 ], for the for the direct oxidation of ethane to acetic acid follows their
oxidation behavior (Table 1 and Figure 6): [ReCl2 {κ3 -HC(pz)3 }(PPh3 )][BF4 ] presents considerably lower
oxidation potential (E1/2 ox = 0.54 V vs. SCE) and leads to higher acetic acid yield (16%) and TON (8)
values than [ReCl3 {κ3 -HC(pz)3 }] (E1/2 ox = 1.14 V vs. SCE; 5% acetic acid yield and TON = 2) [51,52].
Catalysts 2017, 7, 12 Catalysts 2017, 7, 12
13 of 21 13 of 21
Figure 6. Yields of: () acetic acid produced from the one-pot oxidation of ethane catalyzed by
Figure 6. Yields of: () acetic acid produced from the one‐pot oxidation of ethane catalyzed by the the Re(III) complexes vs. their Re(III) oxidation potentials; (•) 6-methylhexanolide obtained from
Re(III) complexes vs. their Re(III) oxidation potentials; () 6‐methylhexanolide obtained from Baeyer‐
Baeyer-Villiger (BV) oxidation of 2-methylcyclohexanone, vs. their Re(VII) oxidation potentials; and (•)
Villiger (BV) oxidation of 2‐methylcyclohexanone, vs. their Re(VII) oxidation potentials; and () 6‐
6-methylhexanolide from BV oxidation of 2-methylcyclohexanone, vs. their Re(III) oxidation potentials.
methylhexanolide from BV oxidation of 2‐methylcyclohexanone, vs. their Re(III) oxidation potentials. 3.3. Baeyer-Villiger Oxidation of Ketones
3.3. Baeyer‐Villiger Oxidation of Ketones The regioselective Baeyer-Villiger (BV) oxidation of 2-methylhexanone to 6-methylhexanolide (as
The regioselective Baeyer‐Villiger (BV) oxidation of 2‐methylhexanone to 6‐methylhexanolide a result of the formal insertion of the oxygen atom between the carbonyl and the more substituted
(as a result of the formal insertion of the oxygen atom between the carbonyl and the more substituted C atom) was found [21] to be favored by strong Lewis acid Re catalysts (Table 1 and Figure 6).
Cαα atom) was found [21] to be favored by strong Lewis acid Re catalysts (Table 1 and Figure 6). The The non-radical mechanism of BV ketone oxidation proceeds via the activation of the ketone, upon
non‐radical mechanism of BV ketone oxidation proceeds via the activation of the ketone, upon coordination to the metal catalyst, to nucleophilic attack of the peroxide oxidant, followed by heterolytic
coordination to the metal catalyst, to nucleophilic attack of the peroxide oxidant, followed by peroxo-bond cleavage and carbanion migration. In fact, for the same Re oxidation state (III (•) or
heterolytic peroxo‐bond cleavage and carbanion migration. In fact, for the same Re oxidation state VII (•), Figure 6) higher lactone yields are obtained when the catalyst presents a higher (more positive)
(III () or VII (), Figure 6) higher lactone yields are obtained when the catalyst presents a higher reduction potential. The electron deficiency of the catalyst (stronger Lewis acid character) activates to
(more positive) reduction potential. The electron deficiency of the catalyst (stronger Lewis acid a greater extent the carbonyl group of the ketone for the nucleophilic attack by hydrogen peroxide.
character) activates to a greater extent the carbonyl group of the ketone for the nucleophilic attack by hydrogen peroxide. 3.4. Oxidation of 1,2-Diols
The Fe(III)/Fe(II) redox potentials of the chloro-iron(III) complexes follow the trend [FeCl3 {κ3 3.4. Oxidation of 1,2‐Diols HC(3-iPrpz)3 }] (3-iPr = iso-propyl group on the 3-position of pyrazole rings) > [FeCl3 {κ3 -HC(pz)3 }]
The Fe(III)/Fe(II) redox potentials of the chloro‐iron(III) complexes follow the trend [FeCl3{κ3‐
> [FeCl3 {κ3 -HC(3,5-Me2 pz)3 }] (Table 1), which represents a decrease in Lewis acidity of the iron(III)
HC(3‐iPrpz)3}] (3‐iPr = iso‐propyl group on the 3‐position of pyrazole rings) > [FeCl
3{κ3‐HC(pz)3}] > center along this series. In [FeCl3 {κ3 -HC(3-iPrpz)3 }], the sterically hindering iso-propyl group weakens
[FeCl3{κ3‐HC(3,5‐Me2pz)3}] (Table 1), which represents a decrease in Lewis acidity of the iron(III) the coordination of nitrogen of pyrazol-1-yl ring conferring an enhanced Lewis acidity of the iron(III)
center along this series. In [FeCl3{κ3‐HC(3‐iPrpz)3}], the sterically hindering iso‐propyl group weakens center. The electron-releasing methyl groups on the pyrazol-1-yl ring in [FeCl3 {κ3 -HC(3,5-Me2 pz)3 }]
the coordination of nitrogen of pyrazol‐1‐yl ring conferring an enhanced Lewis acidity of the iron(III) increase the electron density on pyrazol-1-yl nitrogen and hence decreases the Lewis acidity of the
center. The electron‐releasing methyl groups on the pyrazol‐1‐yl ring in [FeCl3{κ3‐HC(3,5‐Me2pz)3}] iron(III) center [92].
increase the electron density on pyrazol‐1‐yl nitrogen and hence decreases the Lewis acidity of the The catechol dioxygenase activity of the above iron(III) complexes was tested and the
iron(III) center [92]. electrochemical properties of the catecholate adducts of the complexes reveal that a systematic variation
The catechol dioxygenase activity of the above iron(III) complexes was tested and the in the ligand donor atom type significantly influences the Lewis acidity of the iron(III) center and
electrochemical properties of the catecholate adducts of the complexes reveal that a systematic hence the interaction of the complexes with simple and substituted catechols.
variation in the ligand donor atom type significantly influences the Lewis acidity of the iron(III) The rate of oxygenation increases upon increasing the Lewis acidity of the iron(III) center by
center and hence the interaction of the complexes with simple and substituted catechols. modifying the ligand substituents. One of the pyrazolyl arms in the catecholate adducts is sterically
The rate of oxygenation increases upon increasing the Lewis acidity of the iron(III) center by constrained by the 6,6,6-chelate ring system and appears to dissociate from the coordination sphere
modifying the ligand substituents. One of the pyrazolyl arms in the catecholate adducts is sterically upon binding to the catecholate substrate, which is followed by dioxygen attack at the equatorial plane
constrained by the 6,6,6‐chelate ring system and appears to dissociate from the coordination sphere leading to the formation of benzoquinone [92].
upon binding to the catecholate substrate, which is followed by dioxygen attack at the equatorial plane leading to the formation of benzoquinone [92]. Catalysts 2017, 7, 12
Catalysts 2017, 7, 12 14 of 21
14 of 21 3.5. Carboxylation of Alkanes 3.5. Carboxylation of Alkanes
The The catalytic catalytic activity activity of of C‐homoscorpionate C-homoscorpionate V(V) V(V) complexes complexes for for the the one‐pot one-pot carboxylation carboxylation of of
methane to acetic
acetic acid
acid (Scheme was found in accordance with the oforder their methane to
(Scheme
1) 1) was
found
[47][47] to beto inbe accordance
with the
order
their of V(V)
→ V(V) V(IV) reduction potentials, which follows the electron‐donor characters of the scorpionate V(IV)→
reduction
potentials, which follows the electron-donor characters of the scorpionate ligands
ligands and the charge of the complex (see Table 1 and Figure 7). The stronger vanadium(V) Lewis and the charge of the complex (see Table 1 and Figure 7). The stronger vanadium(V) Lewis acid
acid (the easiest to reduce) favors the carboxylation mechanism represented in Scheme 3 and allows (the easiest to reduce) favors the carboxylation mechanism represented in Scheme 3 and allows the
the highest product yield (under the same experimental conditions of the other two V(V) complexes) highest product yield (under the same experimental conditions of the other two V(V) complexes) to be
to be achieved. Moreover, turnover number values follow the yields trend [47]. achieved. Moreover, turnover number values follow the yields trend [47].
Figure 7. Yields of acetic acid produced from the one‐pot carboxylation of methane catalyzed by the Figure 7. Yields of acetic acid produced from the one-pot carboxylation of methane catalyzed by the
{κ33-HC(pz)
‐HC(pz)33}][BF44], [VO22{κ3‐HC(3,5‐Me
V(V) complexes [VO
[VO22{κ
V(V) complexes
], [VO
-HC(3,5-Me22pz)
pz)33}][BF
}][BF44] ]and and [VO22{κ
{κ33‐SO
-SO33C(pz)
C(pz)3}] vs.
3 }]vs. their V(V) to V(IV) reduction potentials. their V(V) to V(IV) reduction potentials.
3.6. Epoxidation of Alkenes 3.6. Epoxidation of Alkenes
For the o-benzoquinonediimine
o‐benzoquinonediimine (bqdi)
(bqdi) Ru(II)
Ru(II) complexes
complexes [RuCl{κ
[RuCl{κ33-HC(pz)
‐HC(pz)33}(bqdi)][ClO
For the
}(bqdi)][ClO44] ] and and
3
3
[Ru(H
4]2, electrochemical and DFT calculations established [89] that the [Ru(H22O){κ
O){κ ‐HC(pz)
-HC(pz)3}(bqdi)][ClO
}(bqdi)][ClO
]
,
electrochemical
and
DFT
calculations
established
[89]
that
the
3
4 2
redox non‐innocent bqdi was stabilized in its fully oxidized quinone state in both the chloro complex redox non-innocent bqdi was stabilized in its fully oxidized quinone state in both the chloro complex
3
+
2+
([Ru{κ
([Ru){κ33‐HC(pz)
([Ru{κ3‐HC(pz)
-HC(pz)33}(bqdi)(Cl)]
}(bqdi)(Cl)]+) )and and the the aqua aqua ([Ru){κ
-HC(pz)33}(bqdi)(H
}(bqdi)(H22O)]
O)]2+) )derivative. derivative.The The chloro chloro
complex exhibits metal based Ru(II)/(III) oxidation and bqdi centered reduction. complex exhibits metal based Ru(II)/(III) oxidation and bqdi centered reduction. 3
The aqua complex [Ru(H
The aqua complex [Ru(H22O){κ
O){κ3‐HC(pz)
-HC(pz)33}(bqdi)][ClO
}(bqdi)][ClO4]42] exhibits two one electron oxidations at 2 exhibits two one electron oxidations at
pH 7, suggesting the formation of a {Ru(IV)=O} species, the supposed active species of the alkene pH 7, suggesting the formation of a {Ru(IV)=O} species, the supposed active species of the alkene
2+
epoxidation cycle (see
(see Scheme
Scheme 5).
5). Thus,
Thus, [Ru(H
[Ru(H22O){κ
O){κ33-HC(pz)
‐HC(pz)33}(bqdi)]
epoxidation catalytic catalytic cycle
}(bqdi)]2+ functions functions as as an an
efficient pre‐catalyst for the selective epoxidation of a wide variety of alkenes in the presence efficient pre-catalyst for the selective epoxidation of a wide variety of alkenes in the presence of of
iodobenzene diacaetate as the sacrificial oxidant. iodobenzene diacaetate as the sacrificial oxidant.
3.7. Redox Potential Parametrization 3.7. Redox Potential Parametrization
The values of the Ru(II/III) oxidation potential (in the range of 0.95–1.37 V vs. SCE, Table 1) of The values of the Ru(II/III) oxidation potential (in the range of 0.95–1.37 V vs. SCE, Table 1) of
[Ru(L)(L′)]X complexes [L = p‐cymene, benzene, hexamethylbenzene (HMB), or cyclooctadiene (cod), [Ru(L)(L0 )]X complexes [L = p-cymene, benzene, hexamethylbenzene (HMB), or cyclooctadiene (cod),
L′ = tris(pyrazol‐1‐yl)methanesulfonate or the 3‐phenylpyrazolyl‐substituted derivative, X = Cl or BF
L0 = tris(pyrazol-1-yl)methanesulfonate or the 3-phenylpyrazolyl-substituted derivative, X = Cl or4] reflect [54] [54]
the electron‐donor characters of their ligands: for BF4 ] reflect
the electron-donor
characters
of their
ligands:
forthe thecationic cationiccomplexes, complexes,with with the the
3
3‐SO3C(pz)3}]++
common [Ru{κ
center, the order of the oxidation potentials follows that (in the opposite common [Ru{κ -SO3 C(pz)3 }] center, the order of the oxidation potentials follows that (in the opposite
direction) the electron‐releasing of the variable
corresponding variable ligand direction) ofof the electron-releasing
charactercharacter of the corresponding
ligand (cymene
> benzene)
(cymene > benzene) as measured by the electrochemical Lever E
L ligand parameter (+1.48 and +1.59 as measured by the electrochemical Lever EL ligand parameter (+1.48
and +1.59 V vs. NHE for cymene
V vs. NHE for cymene and benzene, respectively) [54]. and benzene, respectively) [54].
As mentioned in the introduction, EL is a measure of the electron‐donor character of the ligand, the stronger this character, the lower is EL. Moreover, the experimental oxidation potentials are in accordance with those predicted from the knowledge of EL values for cymene [54], benzene [54] and Catalysts 2017, 7, 12
15 of 21
As mentioned in the introduction, EL is a measure of the electron-donor character of the
ligand, the stronger this character, the lower is EL . Moreover, the experimental oxidation
potentials are in accordance with those predicted from the knowledge of EL values for cymene [54],
benzene [54] and κ3 -SO3 C(pz)3 (Table 2) [52] by applying the Lever method. Accordingly, the
higher oxidation potentials of [Ru{κ3 -SO3 C(3-Phpz)3 }(benzene)]Cl or [RuCl{κ3 -SO3 C(3-Phpz)3 }(cod)],
bearing the 3-phenyl substituted tris(pyrazol-1-yl)methanesulfonate ligand, than those of the
analogous [Ru(benzene){κ3 -SO3 C(pz)3 }]Cl or [RuCl(cod){κ3 -SO3 C(pz)3 }] reflect the expected weaker
electron-donor character of {κ3 -SO3 C(3-Phpz)3 }− ligand in comparison with that of {κ3 -SO3 C(pz)3 }− .
Hence, the former ligand should present a higher EL value than the latter [52] (Table 2).
Table 2. Electrochemical EL Lever ligand parameter for C-homoscorpionate ligands.
Tris(pyrazol-1-yl)methane
HC(pz)3
{SO3 C(pz)3 }−
{SO3 C(3-Phpz)3 }−
a
EL /V vs. SHE a
0.14
−0.09
−0.05
for each coordinated pyrazol-1-yl group.
The electrochemical EL Lever parameters for tris(pyrazol-1-yl)methane ligands [23,51,52,54] to
date, found possible to estimate from the oxidation potential values of C-homoscorpionate complexes,
by applying the linear (valid for octahedral complexes) relationship (1) and considering its extension
to square-planar coordination and to full- and half-sandwich complexes [28,99–103], are presented
in Table 2. These values correspond to partial EL parameters assigned to each metal ligated arm
(2-electron-donor) of the scorpionate ligand. Thus, the overall EL value of a scorpionate ligand will
depend on its coordination mode to the metal center in a complex.
Since the EL parameter is a measure of the electron donor character of a ligand (the lower
the parameter value, the stronger is that character), each ligated pyrazol-1-yl arm in {SO3 C(pz)3 }−
(EL = −0.09 V vs. SHE, Table 2) is clearly a stronger electron donor than in HC(pz)3 (EL = 0.14 V vs.
SHE), indicating a much stronger electron-releasing ability of the anionic CSO3 − group at {SO3 C(pz)3 }−
than the methine HC group in the neutral HC(pz)3 . That is consistent with the above reported
electrochemical behavior for V, Re, and Au complexes. Moreover, the value of –0.05 V vs. SHE (for
each coordinating pyrazolyl arm) agrees with the expected slightly weaker electron-donor character of
SO3 C(3-Phpz)3 − relative to SO3 C(pz)3 − due to the phenyl substituent at the pyrazol-1-yl rings in the
former ligand.
The above two series Ru(II) complexes, bearing the tris(pyrazol-1-yl)methanesulfonate ligand
and its 3-phenyl substituted derivative, have not yet been tested as catalysts for partial oxidation
reactions. Nevertheless, based on the reported electrochemical studies [54] we would expect
a better oxidative catalytic performance for the tris(pyrazol-1-yl)methanesulfonate complexes
[Ru(p-cymene){κ3 -SO3 C(pz)3 }]Cl and [Ru(cod)Cl{κ3 -SO3 C(pz)3 }].
A comparison of the effect of HC(pz)3 or HB(pz)3 ligands on the redox potential of a metal complex
was reported for acetonitrile-Ru(II) complexes [93]. [Ru{κ3 -HB(3,5-Me2 pz)3 }(NCCH3 )3 ][OTf] exhibits
a higher Ru(III)/(II) potential than its carbon analogue [Ru{κ3 -HC(3,5-Me2 pz)3 }(NCCH3 )3 ][BF4 ]2
(0.59 and 0.42 V vs. SCE, respectively), indicating that the charged {κ3 -HB(3,5-Me2 pz)3 }− ligand
stabilizes Ru(III) relative to Ru(II) compared to the neutral {κ3 -HC(3,5-Me2 pz)3 }. In contrast, the
Ru(III)/(II) reduction potential observed for the [Ru{κ3 -HB(3,5-Ph2 pz)3 }(NCCH3 )3 ][BF4 ] complex
(0.57 V vs. SCE) is lower than the same potential for the [Ru{κ3 -HC(3,5-Ph2 pz)3 }(NCCH3 )3 ][BF4 ]2
complex (0.71 V vs. SCE, Table 1), indicating that ligand charge is not as significant a factor as
steric in determining the stability of ruthenium oxidation states for complexes with these bulky
ligands. Since the redox potential of [Ru{κ3 -HB(3,5-Ph2 pz)3 }(NCCH3 )3 ][BF4 ] complex is smaller
than that of the [Ru{κ3 -HB(3,5-Me2 pz)3 }(NCCH3 )3 ][OTf] complex, but the reverse is true for the
[Ru{κ3 -HC(3,5-Ph2 pz)3 }(NCCH3 )3 ][BF4 ]2 and [Ru{κ3 -HC(3,5-Me2 pz)3 }(NCCH3 )3 ][BF4 ]2 complexes,
Catalysts 2017, 7, 12
16 of 21
both electronic and steric factors of these ligands affect the redox potentials of their Ru(II) complexes.
Overall, Ru(II) coordination by negatively charged {κ3 -HB(3,5-R2 pz)3 }− (R = Me or Ph) or neutral
{κ3 -HC(3,5-R2 pz)3 } ligands with varying steric bulk alters the Ru(III)/(II) potential by over 400
mV. The ability to alter the stability of ruthenium +2 or +3 oxidation states may be used to tune
catalytic reactions.
The electronic and structural properties of scorpionate ligands, such as poly(pyrazol-1-yl)methane
ligands, play an important role in the ability of several transition metal complexes to mediate C–H
activation and functionalization as well as other partial oxidations. Thus, the knowledge of the redox
behavior of a certain C-homoscorpionate catalyst, as well as its relationship with the structure of the
catalyst, may allow tailoring of its structural design to present a favorable value of potential to enhance
its catalytic performance. Moreover, tris(pyrazol-1-yl)methane ligands may act as more than simple
spectators during chemical reactions experienced by their metal complexes, and have an important
influence on their reactivity by means of temporary changes of denticity.
Of course, other factors are involved in the catalytic activity exhibited by the metal complex.
Importantly, the tripodal C-scorpionate ligand, bearing three pyrazol-1-yl moieties (via their N atoms)
is found to assist proton-transfer steps (see Scheme 4) that are involved in key catalytic oxidation
processes. Such factors should additionally be considered in the design of a catalyst with expected
improved activity for the above oxidation reactions.
Acknowledgments: The author gratefully acknowledges all the co-authors cited in the joint publications.
The work in this area has been partially supported by the Fundação para a Ciência e a Tecnologia (FCT), Portugal,
and its projects PTDC/QEQ-ERQ/1648/2014 and UID/QUI/00100/2013.
Conflicts of Interest: The author declares no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the
decision to publish the results.
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